1. Field of the Invention
The invention generally relates to a molding additive composition comprising a mixture of a low profile additive and a surfactant additive containing a silicon-oxyalkylene copolymer flow control agent and surface modifying agent. The invention also relates to a molding composition comprising a polyester molding resin, a low profile additive, and a silicon-oxyalkylene copolymer flow control agent and a surface modifying agent. A preferred composition of the invention contains a crosslinking monomer. In its most preferred embodiment, the invention specifically relates to a molding composition comprising a polyester molding resin, a crosslinking monomer, a low profile additive, a reinforcing fiber, and a silicon oxyalkylene copolymer flow control agent and surface modifying agent.
2. Prior Art
Low profile additives have been used commercially for about 30 years. They are valuable additives to thermosetting molding compositions that are used to generate molded articles that possess minimal shrinkage. Generally, the low profile additives are thermoplastic materials which at the curing temperature of the thermosetting resin, have a limited degree of compatibility with the cured resin. Consequently, the additive separates from a condition of solution with the cured molding resin, creating void spaces in the surface of the molded article which occupy the space in the mold created by shrinkage of the resin due to the thermosetting crosslinking reaction.
Looking at the low profile additive development in a more limited sense, it relates to a technical improvement that has made a significant contribution to commercial polyester molding technology. The use of low profile additives to reduce shrinkage during the curing reaction result in improving dimensional stability and surface smoothness of unsaturated polyester resins. Low profile additives are typically thermoplastic polymers such as vinyl acetate polymers, polystyrene, acrylic polymers, polyurethanes, aliphatic polyesters (such as derived from polycaprolactones and aliphatic polyesters derived from dicarboxylic acids and aliphatic diols).
There are a number of theories that seek to explain the low profile or anti-shrinkage action of these polymers, but the one that seems to best explain the phenomenon relative to unsaturated polyesters blended with styrene monomer is as follows.
The low profile additive is at least partly soluble in the uncured polyester/styrene solution. As the polyester/styrene mixture crosslinks, the thermoplastic polymer becomes incompatible or less soluble and at least partly comes out of solution. This action causes a volume expansion that compensates for the shrinkage that occurs when the polyester/styrene mixture crosslinks.
The development of low-profile thermosetting resin compounds, such as unsaturated polyester compounds, has led to a wide acceptances of these materials by the transportation and other industries because of their good surface appearance, dimensional stability, physical properties, assembly consolidation and potential weight savings. However, as new applications have developed, standards have been raised, making it desirable to have even better surface appearance and the elimination of ripples and waviness that sometimes develop, particularly in relatively large appearance sensitive areas.
Developments have been made in this art which are related to special compositions and special problems. For example, U.S. Pat. No. 4,032,494, patented June 28, 1977, uses liquid siloxanes as additives, in polyester composites containing vinylacetate polymers to reduce the water absorption of the cured polyester composite. The siloxanes illustrated in the patent all possess the silicone structure, to the essential exclusion of other structures, though the patent fails to indicate that other structures are not embodied by the invention of the patent.
In relating to a pigmentability problem, U.S. Pat. No. 4,555,534, patented Nov. 26, 1985, embraces a curable molding composition containing: (a) a polyester resin; (b) an olefinically unsaturated monomer; (c) a thickening agent comprising an oxide or hydroxide of a metal of Group I, II, or III of the Periodic Table; (d) a pigment; (e) a carboxylated vinyl acetate polymer low profile additive; and (f) a surface active compound that is employed in an amount sufficient to enhance the uniformity of the pigmentation of the cured composite produced from said curable molding composition.
The patent describes a wide variety of surface active compounds that enhance the uniformity of the pigmentation of the cured composite produced from the curable molding compositions. The patent also characterizes a number of surface active compounds that fail to enhance the uniformity of the pigmentation of the cured composite produced from the curable molding compositions. For example, Table XV, at column 16 of the patent, lists a number of surface active compounds that failed to enhance the uniformity of pigmentation. Those were the surfactants that yielded a "pigmentability rating" of less than 3, and appear to comprise those surfactants that were not cited in Table XVI of the patent. Table XVII, at Col. 17, indicates that the control, using a different low profile additive than the one used with Tables XV and XVI, had a "pigmentability rating" of 1, and Table XVIII lists surfactants which gave a pigmentability rating of 2 or more, "which means that they improved the pigmentability compared with the formulation containing no surfactant." Of the about 94 illustrations of surfactants mentioned in Table XV, L-5420 and L-5303, Column 16, are siloxane-polyoxyalkylene copolymers. They gave pigmentability ratings of 2, meaning, according to the discussions before Table XVI, they failed to improved the pigmentability. There is no indication as to how they would have fared if used with the low profile system that was employed with Tables XVII and XVIII.
None of these materials described in U.S. Pat. No. 4,555,534 are shown to be flow control agents or surface modifying additives. They are described to enhance pigmentability, which means the composition has to possess a pigment. This patent is restricted in its teachings to compositions containing pigments.
The invention is concerned with an improvement in thermosetting resin molding compositions and additive compositions which may be employed in making effective thermosetting resin molding compositions. The present invention contemplates molding compositions suitable for compression, casting, transfer, injection, pultrusion, resin transfer, vacuum forming, and the like, molding techniques. The present invention is directed to thermoset molding systems possessing enhanced flow characteristics and surface control.
Fiber reinforced plastics (FRP) are being considered for many new automotive applications. The quality of these new FRP parts will be affected by the flow of the molding compound in the press. A screening evaluation of thirty potential flow additives in bulk molding composition has indicated that among the best candidates identified for flow improvement are silicone surfactants of the type characterized herein. Some have shown to be very effective in raising tensile strength (from 25-60%).
Some of the problems associated with fiber reinforced plastics (FRP) are porosity, blisters, non-fills, surface smoothness, and strength properties. The degree to which any of the above become problems could depend on the flow of the molding compound during the heterogeneous mixture of resin, filler and fiberglass. The flow comes from the resin portion (30 weight percent) which must distribute the filler (about 42 weight percent) and the fiberglass about 30 weight percent) uniformly over the molded fiberglass part.
As these materials flow, they push out the air which, if not removed, could cause porosity, blisters and poor Physicals. How smoothly and evenly these materials flow determines many times how smooth the surface will be. Some of the factors that influence flow are listed below:
(a) Molding formula
(b) Filler content
(c) Fiberglass length and concentration
(d) Polyester
(e) Peroxide initiator
(f) Inhibitor level
It has been determined that many of the problems concerning the moldability of fiber reinforced plastics are ameliorated by the provision of the flow additives of the present invention. The flow improvements provided by the present invention allows the invention to help in meeting the increase in quality standards for fiber reinforced molded parts, especially as set by automotive manufacturers and owing to the fact that the new parts being molded
1. Are larger and more complex
2. Have large, flat surfaces which require smoother surfaces
3. Use polyester resin systems which provide enhanced toughness but which do not flow as well, and
4. Require higher glass contents to get desired strength properties.
The present invention contemplates the use of resin systems that employ low profile additives in combination with a variety of thermosetting resins, that is, resins that cure by crosslinking through functional groups in the polymeric resin. The greatest proportion of the resin systems employ unsaturated polyester resins. Other resins include, e.g., phenolic resins, epoxy resins, urethane resins, and the like.
It is an object of the invention to provide a molding composition that provides one or more of the following advantages: improve flow during molding; improved physicals for the molded product; smoother surfaces for the molded product, i.e., better replication of the mold and the mold's dimensions; and molded fiber reinforced plastic parts that have less shrinkage.
Another object of the present invention is the fact that none of the aforementioned compositions require the presence of a pigment. In the usual case in practicing the invention, the compositions will be free of pigments. Moreover, it is not the intent of the invention to provide improved pigmentability or reduced water absorbency.